Accompanying increasing demands in recent years for greater energy savings during image formation, efforts have been made to lower the toner fixation temperature. Additional reductions in the fixation temperature achieved through the use of low softening temperature polyesters have been proposed as one approach here.
However, due to the low softening temperature, under conditions of quiescence, e.g., during storage or transport, the toner can undergo melt agglomeration and blocking can be produced.
As a means for balancing the blocking resistance with the low-temperature fixability, patent Literature 1 to 3 teach art that uses a crystalline resin that has a sharp melt property, i.e., its viscosity undergoes a large decline when the melting point is exceeded.
However, a major problem occurs when crystalline polyester, which is a crystalline resin, is used by itself as the binder resin, i.e., due to the low electrical resistance of crystalline polyesters, the charge on the toner gradually leaks away after triboelectric charging.
Patent Literature 4 describes a toner that has a reduced amount of addition of crystalline polyester and that uses a mixture of a crystalline polyester and an amorphous resin readily compatible therewith.
However, this is a toner that contains an amorphous resin along with crystalline polyester as the binder resin, and the following problem can occur when readily compatible resins are combined with each other.
When, during toner production, a step of melting by heating to at least the melting point of the crystalline polyester is carried out, or a step of dissolving the crystalline polyester using an organic solvent is carried out, the amorphous resin and the crystalline polyester remain present miscibilized in the toner. As a result, plasticization of the amorphous resin (that is, a lowering of the glass transition temperature) is induced, and as a consequence, while the sharp melt property is excellent, the charging performance and heat-resistant storability are inadequate and deterioration may occur.
In the case, on the other hand, of a toner that uses a mixture of crystalline polyester and an amorphous resin poorly compatible therewith, the resins are poorly compatible with each other and the following problem can be produced.
During toner production, after the step of melting by heating to at least the melting point of the crystalline polyester has been carried out, or after the step of dissolving the crystalline polyester using an organic solvent has been carried out, the amorphous resin and the crystalline polyester are phase-separated from each other and a matrix-domain structure is spontaneously formed in correspondence to the compatibility of the resins. As a result, plasticization of the amorphous resin (that is, a lowering of the glass transition temperature) is not induced and, while the charging performance and heat-resistant storability are excellent, the low-temperature fixability is inadequate due to the low compatibility.
As a method for inducing phase separation of a mutually dissolved crystalline polyester and amorphous resin when readily compatible resins have been combined with each other, Patent Literature 5 teaches a method in which phase separation is induced through crystallization of the crystalline polyester achieved by providing an annealing step in which crystallization is promoted by heat-treating the toner at a temperature near the melting point but not above the melting point of the crystalline polyester.
Patent Literature 6 describes the following method as a method for suppressing compatibilization during toner production: the crystalline polyester is recrystallized by dissolution in a solvent and cooling; the crystalline polyester is subsequently mechanically pulverized and dispersed in a solvent; a toner constituent component containing an amorphous resin is then dissolved or dispersed in the solvent; and toner is obtained through a granulating step.
Patent Literature 7 describes a toner in which the low-temperature fixability is made to coexist in good balance with the storability by regulating the domain diameter of the crystalline polyester present in the toner.
Patent Literature 8 describes a toner that exhibits an excellent low-temperature fixability and an excellent releasability during fixing, which is brought about by regulating the aspect ratio of the domains formed by the crystalline resin incorporated in the toner.